The invention relates to a preparation process for pulverulent coating materials using a low molecular mass inert compound as auxiliary.
Of the large number of known powder coating preparation processes, the processes which find particularly broad application are those in which the ready-formulated powder coating material is extruded in the form of a pastelike melt and is brought to the desired particle size, after cooling of the melt and coarse comminution, by fine milling and subsequent sieving. The following process steps in particular are common for coloured powder coating materials:
1. The constituents of the powder coating material are subjected to thorough initial mixing in dry form as solids in the required proportions.
2. The mixture is melted in an extruder at a very low temperature in order to avoid premature cross-linking, and is thoroughly mixed. In the course of this mixing, binder and hardener are plasticized and wet the pigments and fillers.
1. The optionally coloured extrudate obtained is rolled out into a thin layer, cooled and crushed to give coarse granules.
2. The granules are milled in a mill to give the finished powder coating material.
In general, the unavoidable fine fraction  less than 10 xcexcm in the milling process is removed in a subsequent screening process. The resulting powder typically has a mean particle size of from 40 to 70 xcexcm.
(Compare Ullmanns Enzyklopxc3xa4die der technischen Chemie, Volume 15, page 680, 1978, Verlag Chemie Weinheim, and the monograph xe2x80x9cThe Science of powder coatingsxe2x80x9d Volume 1 and 2 (Editor D. A. Bate, London 1990)). In the case of these processes there may be difficulties and/or laborious subsequent operations, especially in regard to the extrusion and milling procedures. In the case of highly reactive binder mixtures, the extrusion procedure may take place too slowly so that partial gelling occurs, which no longer permits use of the product for coatings. Limiting the residence time in the extruder, on the other hand, results in non-optimum dispersion of the pigments in the binder. In carrying out coating with powder coating material, this poor dispersion leads to a relatively poor hiding power which has to be compensated by a relatively high proportion of pigments. The extrusion step also imposes a limitation in respect of the binders that can be employed, since it is possible to operate only within a certain viscosity range. For example, newly developed crystalline resins, which above the melting point possess a very low melt viscosity and give rise to outstanding powder coating materials, cannot be processed in an extruder. The use of mixtures of resins having very different viscosities is also of only limited feasibility in an extruder, owing to the poor homogenization of such systems. The broad particle size spectrum which forms in the course of milling is in the range, for example, of from 0.1 to 500 xcexcm and requires additional sieving and milling procedures for specific applications. The fines which are produced, moreover, are disadvantageous for both health and processing reasons.
Also known are processes for preparing powder coating materials, in which powder coating melts are sprayed (DE-A-22 33 138, EP-A-0 537 233). The melting of the base resin and hardener components may lead to thermal stressing of the powder coating composition. Reducing the contact time between base resin and hardener in the melted state is possible only at great technical expense.
It is also known to employ low molecular mass inert compounds in the form of compressible fluids as auxiliaries for the preparation of powders and powder coating materials. Thus in accordance with EP-A0 157 827, WO 95/34606 and EP-A-0 720 999 the components of the powder coating composition are dissolved in a supercritical fluid and the resulting solution is sprayed with pressure release.
Cooling induced by the procedure of pressure release causes the formation of the powder particles. This pressure release can be carried out spontaneously or as a function of time, continuously or in stages. It is possible to obtain particle sizes of from 5 to 150 xcexcm, but also very fine particles in the range from 1 to 5 xcexcm or less, by using appropriate nozzles.
A disadvantage of these processes is that the concerned components are in many cases of only slight solubility in supercritical fluids. High pressures and large amounts of gas are necessary in order to dissolve the solids in the supercritical fluid.
EP-A-0 669 858 and EP-A-0 661 091 relate to preparation processes for coating powders, in which the solid starting substances are intended to be insoluble in the supercritical fluid under process conditions. One of the purposes of this is to avoid losses due to transportation of starting material out of the pressure release vessel. According to EP-A-0 669 858, the solid starting substances are mixed homogeneously in the supercritical fluid and then this mixture is relieved of its pressure, whereas in accordance with EP-A0 666 091 the powder components are melted and the liquid material is dispersed in an appropriate fluid.
With processes of this kind there may be problems in terms of the homogeneity of the mixtures.
Also known are processes in which powder preparation takes place from solvent-containing compositions, using compressible fluids, by pressure release of the solution and simultaneous evaporation of the solvent, examples being EP-A-0 711 586 and JP 8-104 830. The solvents used should be partially miscible with the solid components and should have a high volatility rate.
The use of organic solvents and the efforts involved in recovering these solvents render these processes disadvantageous.
To avoid high pressures and amounts of gas, and also the use of organic solvent and the associated expenditure for its recovery, a process has been developed, in accordance with WO 95/21688, in which the compressible fluid is dissolved under pressure in the substance mixture that is to be treated and then the solution is relieved of its pressure. For this purpose, the solid components are melted and an appropriate fluid is dissolved under pressure therein.
This process relates to the preparation of particles and powders, examples being starting materials for the production of emulsifiers and detergents, and also pharmaceutical active ingredients. Where mixtures of different substances are processed by this process, these mixtures are first of all prepared and then inserted into the process. The preparation of reactive powder coating materials is not addressed. A mixture of different powder coating components comprising base resins, hardeners and pigments/fillers and also further coatings additives may lead to unwanted chemical reactions between base resin and hardener which influence the properties of the powder coating compositions unsuccessfully.
The object of the invention is to provide a process which makes it possible to prepare powder coatings continuously or batchwise and in a simple manner while avoiding strong thermal stress. Furthermore the object of the present invention is the preparation of homogeneous powder coating formulations by spraying of gas containing melts without premature chemical reaction between the powder coating components. The process is intended to yield homogeneous powder coating particles with an adjustable mean particle size in the range from 10 to 80 xcexcm and with a narrow particle size distribution and to permit the use of raw materials within a wide viscosity range. In addition, the processing of powder coating compositions with a low crosslinking temperature should be possible. At the same time, the laborious removal of solvents and the use of high pressures, in comparison with conventional processes, is to be avoided.
It has been found that this object can be achieved by the process which forms the subject of the invention, namely a process for preparing powder coating formulations by pressure release of a composition comprising a low molecular mass inert compound, with release of the low molecular mass inert compound as a gas or vapour, which is characterized in that one or more base resins and one or more hardeners for the base resins, or one or more self-curing base resins, and, if desired, further conventional coatings components, such as additives and, in the case of coloured powder coating materials, pigments, dyes and, if desired, fillers are converted together or separately into a flowable form by heating, the low molecular mass inert compound is dissolved under pressure at least in one of the base resin or hardener components employed, either before these components are combined with the other components or in the already prepared mixture, the complete mixture is homogenized, and then the mixture of all components is released from pressure, with cooling and formation of a powder having a mean particle size of between 10 and 200 xcexcm and a narrow particle size distribution.